Electronic switches, consisting of a device that can be alternately turned on and turned off, are commonly used in various applications to make and break selectively a connection between two circuit branches in response to a command signal. An example of such applications are the converter circuits that are able to convert a DC voltage into an alternating voltage (inverter) used, for example, in the speed control devices of asynchronous electric motors.
The switching speed of an electronic switch is limited by the fact that, during turn-off and turn-on, i.e. during the passage from the conducting state to the cut-off state and vice versa, a transitory condition of excess voltage (overshoot) occurs across the power terminals of the electronic switch. Switching necessarily involves a high variation over time of the current passing through the device (dI/dt), that in turn generates an excess voltage on the parasitic inductances of the connections, including the connections inside the device. This excess voltage can even be several times higher than that of the electronic switch's power supply and can reach values and durations that cause the safety limits determined by the switch's structural characteristics to be exceeded. It should be noted that, although the problem can be minimised by careful design of the various electrical connections, it cannot be completely eliminated because of the internal inductances of the components themselves. This problem is particularly acute in high power applications, in which the operating voltage may exceed a thousand V and the operating current may reach some thousands of A.
A well-known technique in the state of the art for reducing the effects of this transitory condition (typically used with IGBT, BJT and MOSFET devices) consists of driving the electronic switch with a high resistance by connecting an appropriate resistor to a control terminal thereof. With this arrangement the time constant RC of the input circuit of the electronic switch increases, thus reducing its switching speed and, consequently, the level of voltage overshoot during switching.
However, this known solution, by reducing the switching speed of the electronic switch, prevents the latter from being used in high frequency applications, in which it is made to switch rapidly between its states of conduction and cut-off. In any case, prolonging the switching time causes an increase in dissipated power from the electronic switch during this phase.
Moreover, this solution is not able to limit the voltage overshoots in particular operating situations, such as, for example, in the case in which the electronic switch must be turned off following a short circuit on the load applied to one of its power terminals. It is therefore necessary to use additional protection circuits (clamps, snubbers), with a consequent increase in cost, in complexity and in the space occupied by the electronic switch.